Fix some instances where CAM rescans get hung up or take a long time to complete.
Fix some instances where CAM rescans get hung up or take a long time to
complete.

Also, allow xpt_rescan() to rescan a LUN instead of a full bus.

MFC after:	3 days
Sponsored by:	Copan Systems, Inc.

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase